Long-distance, high-speed, and large-capacity optical communication becomes essential with the recent increase in data traffic, and construction of a WDM network is progressing. In the WDM network, a plurality of light transmitting apparatuses corresponding to different wavelengths is required, and inventory and type control of the light transmitting apparatuses becomes complicated. A light transmitting apparatus that uses a wavelength-tunable light source, a semiconductor laser (LD) array light source, or the like is a key device effective for inventory and type reduction. In particular, a light transmitting apparatus integrated with a light source and an electro-absorption optical modulator (hereinafter, referred to as EA modulator) disclosed in Patent Document 1 below is effective for miniaturization, and is promising for use as a small size TOSA (Transmitter Optical Sub Assembly) that is mounted on a pluggable module that complies with the standard of XFP (10 Gbit/s small Form-factor pluggable). Pluggable means that the optical module can be taken out and inserted as is in an energized state, with respect to a slot of an optical transmitter.
In an optical communication system that uses the above-described light transmitting apparatus, an average optical output power of the light transmitting apparatus needs to be made constant from a viewpoint of stability of the system. However, in the light transmitting apparatus for a small size TOSA, it is difficult to mount an optical detector (hereinafter, referred to as a monitor PD) for monitoring the optical output power, due to mounting space. Therefore, as an alternative means for the monitor PD, for example in Patent Documents 2 and 3 below, a monitoring method of the optical output power that uses photoelectric current (hereinafter, referred to as EA photoelectric current) generated due to optical absorption by the EA modulator has been proposed. Specifically, in an light transmitting apparatus integrating a light source and the EA modulator, a mean value of the EA photoelectric current is monitored, and a drive current of the light source or a bias voltage of the EA modulator (hereinafter, referred to as EA bias) is controlled so that the monitored value becomes constant at a preset reference value, to thereby control the optical output power to be constant.
Moreover, as a conventional art relating to general control of the light transmitting apparatus integrating the light source and the EA modulator, for example, techniques described in Patent Documents 4 to 7 below are known. Specifically, Patent Document 4 represents where, for a light transmitting apparatus having an LD array light source and an EA modulator, temperature control and EA bias control are performed so that a difference between oscillation wavelength of the light source and bandgap wavelength of the EA modulator becomes constant. Patent Document 5, represents a technique where, for a light transmitting apparatus having an wavelength-tunable light source and an EA modulator, in addition to automatic power control (APC) by back power monitoring of the light source, in order to stabilize the optical output power by eliminating fluctuations in the optical output power due to a temperature change when the wavelength is tunable, an output of an EA modulator is branched and monitored to feed-back control the EA bias according to a result thereof. Patent Document 6 represents where, for a light transmitting apparatus integrating a wavelength-tunable light source and an EA modulator, a drive current of a light source is changed according to a modulation signal, and a chirp is counterbalanced by providing a chirp opposite to the chirp generated in the EA modulator, to the light source, to thereby realize long-distance transmission. Patent Document 7 represents where, for a light transmitting apparatus having a light source and an EA modulator, a drive condition of the EA modulator is adjusted so that variations in the optical frequency of the optical output become an optimum.
Patent Document 1: Japanese Patent No. 2891741
Patent Document 2: Japanese Patent No. 2616206
Patent Document 3: Japanese Laid-open Patent Publication No. 2000-292756
Patent Document 4: Japanese Laid-open Patent Publication No. 2001-144367
Patent Document 5: Japanese Laid-open Patent Publication No. 2001-333020
Patent Document 6: Japanese Patent No. 3453406
Patent Document 7: Japanese Patent No. 3333133
Incidentally in a conventional light transmitting apparatus having the above-described wavelength-tunable light source, or LD array light source and EA modulator, when the monitoring method of the optical output power using the EA photoelectric current is applied, the EA photoelectric current also varies depending on wavelength as illustrated in FIG. 11, for example, due to wavelength dependency of extinction characteristics of the EA modulator as illustrated in FIG. 10. In the figure, λ1, λ2, and λ3 respectively denote a wavelength of light input to the EA modulator, where λ1<λ2<λ3. Therefore, when the drive current of the light source is controlled by using a reference value of the EA photoelectric current common to all the wavelengths as in the conventional art, the optical output power of the light source (optical input power of the EA modulator) varies depending on the wavelength, and as a result, the optical output power of the EA modulator varies as illustrated in FIG. 12. Fluctuations in the optical output power increase as a wavelength-tunable range of the light input to the EA modulator becomes wide, so that there is a problem in that these can no longer be accepted as optical output power fluctuations of the light transmitting apparatus.
Moreover, the above optical output power fluctuations at the time of applying feed-back control based on monitoring of the EA photoelectric current, occur not only due to the wavelength dependency of the EA photoelectric current but also, for example, due to temperature dependency of the EA photoelectric current. That is to say, due to the temperature dependency of the extinction characteristics of the EA modulator as illustrated in FIG. 13, the EA photoelectric current also fluctuates depending on the temperature (T1<T2<T3), as illustrated in FIG. 14. Therefore, if the drive current of the light source is controlled by using the reference value of the EA photoelectric current common to all temperatures, the optical output power of the light source fluctuates depending on the temperature, and as a result, as illustrated in FIG. 15, the optical output power of the EA modulator fluctuates, thereby causing the same problem as for the case of the aforementioned wavelength dependency.
By combining the above-described conventional art, and monitoring the EA photoelectric current, and controlling the EA bias so that the monitor value becomes constant, the optical output power fluctuations of the light transmitting apparatus can be suppressed. However, as described in Patent Documents 4, 6, and 7, the EA bias is a parameter for controlling the chirp, and the EA bias needs to be controlled so that an appropriate chirp characteristic can be obtained, in order to realize long-distance transmission. For example, in long-distance transmission exceeding 80 km, if the EA bias is controlled to suppress the optical output power fluctuations, transmission degradation may be caused due to a change of the chirp characteristic.